First it should be noted that sign and trades are nothing more than a fantasy of teams with impending UFAs - they don’t happen in the real world. So any team that trades for Karlsson will be trading for a single year of his services, which means we’re talking a rental price, not an extended price. The bidding will determine how high that price goes but nobody is going to offer an elite young talent still on his ELC like Point or Sergachev - that’s just not going to happen.
As far as Cirelli is concerned, while I certainly wouldn’t say he’s untouchable for Karlsson, it also doesn’t make a great deal of sense for the Lightning to trade him. The Lightning have two legitimate 1Cs in Point and Stamkos and a solid 2C in Johnson. But Johnson is not well suited to lower line duty and has been moved to 2RW for that reason. And after Cirelli the pickings are pretty slim at the moment for lower line centers. Paquette is our 4C but he spends half the year out with injuries and the other half playing through them. Conacher is not a guy you want in the lineup on a day-to-day basis. Peca just left in free agency. Howden was traded to acquire McDonagh. Stephens will probably see some NHL time next season but he’s ideally going to be Paquette’s replacement on the fourth line. And Lipanov and ABB aren’t ready yet. So trading Cirelli for a rental no matter how good just doesn’t make a ton of sense for us given our current organizational center depth, especially if you think his offensive production this past season wasn’t a fluke (personally I’m not sure he can maintain that pace long term but others are more bullish on his offensive potential.)
Now all that said if you offer me a year of Karlsson for Cirelli then I’m obviously going to have a hard time turning that down. But I can understand why people would be hesitant.